linux-kernel - Re: [PATCH 2/2] irq: Cleanup context state transitions in irq

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Date:	Fri, 22 Feb 2013 13:08:17 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] irq: Cleanup context state transitions in irq_exit()

On Fri, Feb 22, 2013 at 7:06 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> I prefer to let you guys have the final word on this patch. Whether you
>> apply it or not, I fear I'll never be entirely happy either way :)
>> That's the sad fate of dealing with circular dependencies...
>
> plus the butt ugly softirq semantics or the lack thereof ...

The softirq semantics are perfectly fine. Don't blame softirq for the
fact that irq_exit() has had shit-for-brains for a long time.

Just move the whole "invoke_softirq()" thing down to *after* the
tick_nohz_irq_exit() stuff. And that "wakeup_softirqd()" is garbage
too, since the whole thing should only be used for the
"force_irqthreads" case (which invoke_softirq()" got right.

And get rid of that final

 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
        local_irq_restore(flags);
 #endif

because even if the architecture enters irq_exit() with interrupts
enabled, we should damn well exit with them disabled so that there are
no races with new recursive interrupts (other than the ones that
wakeup_softirqd already handled).

In other words, I think all those special cases are indeed indicative
of something being wrong, but that "something" is not the softirq
code. Don't blame that. Blame the fact that irq_exit() is simply
written wrong. The softirq code should be done last (it used to be
done from the asm code), and the whole comment about
tick_nohz_irq_exit() perhaps changing something looks like pure and
utter garbage.

Don't blame the wrong code here.

               Linus
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